function y=dE(x0,y0,z0,b)
y=abs(-2*x0^2+2*y0*(b-y0)-60*(15-z0))/((sqrt((4*x0^2+4*y0^2+3600)*(x0^2+(b-y0)^2+(15-z0)^2)))*(x0^2+(b-y0)^2+(15-z0)^2));